C. N. Gibbins

Groundwater–surface water interactions in upland Scottish rivers: hydrological, hydrochemical and ecological implications

Synopsis Contrary to previous hydrogeological assumptions, we now know that drift deposits and fracture systems in crystalline rocks can constitute important aquifers in the Scottish Highlands and other montane environments. Groundwater from these aquifers usually has an important influence on the hydrology, hydrochemistry and ecology of upland river systems. Tracer-based research in the Girnock burn catchment in the Cairngorms revealed that groundwater comprises at least 30% of annual runoff. Groundwater often enters stream channels via drift deposits in valley bottom areas, which appear to be fed from recharge areas on the catchment interfluves. A range of groundwater sources exist in the catchment reflecting the complex solid and drift geology. These account for spatial differences in stream hydrochemistry and the spatial delineation of groundwater discharges to rivers and riparian zones. Areas where groundwaters enter the stream channel directly can have profound ecological implications. Most obvious are low rates of salmonid egg survival where chemically reduced groundwater discharges through the hyporheic zone. However, it is argued that only further research will reveal the full significance of groundwater–surface water interactions to the ecological status of Scottish rivers.

Water quality in the Scottish uplands: a hydrological perspective on catchment hydrochemistry

Land above 300 m covers approximately 75% of the surface of Scotland and most of the nations major river systems have their headwaters in this upland environment. The hydrological characteristics of the uplands exert an important influence on the hydrochemistry of both headwater streams and downstream river systems. Thus, many of the spatial and temporal patterns in the chemical quality of surface waters are mediated by hydrological processes that route precipitation through upland catchments. These hydrological pathways also have an important influence on how the hydrochemistry of upland streams is responding to increasing pressures from environmental changes at the global and regional scales. At the present time, atmospheric deposition remains an issue in many parts of the Scottish uplands, where critical loads of acidity are exceeded, particularly in areas affected by increasing N deposition. Moreover, climatic change forecasts predict increasingly wetter, warmer and more seasonal conditions, which may modify the hydrochemical regimes of many river systems, particularly those with a strong snowmelt component. On a more localised scale, land management practices, including felling of commercial forests, expansion of native woodlands, agricultural decline and moorland management all have implications for the freshwater environment. Moreover, increasing public access to upland areas for a range of recreational activities have implications for water quality. Understanding the hydrology of the uplands, through integrated field and modelling studies, particularly of the hydrological pathways that regulate chemical transfers to streamwaters, will remain an important research frontier for the foreseeable future.

Salmonid habitat modelling studies and their contribution to the development of an ecologically acceptable release policy for Kielder Reservoir, North-east England

1. Kielder Reservoir regulates the Rivers North Tyne and Tyne. It provides a regular supply of water for downstream users, supports abstractions for a major water transfer scheme and provides hydroelectric power (HEP). Kielders release regime typically alternates between a 1.3 m3 s−1 compensation flow and 10–15 m3 s−1 HEP releases of between 3 and 7 days in duration. Occasionally releases of up to 30 m3 s−1 are made for the purpose of encouraging fish runs, for recreational events or to help in water quality management. The impacts of this release regime on Atlantic salmon (Salmo salar) and brown trout (S. trutta) habitat at four sites on the North Tyne are assessed and alternative regimes, designed to minimize impacts, are presented. 2. There is no evidence that the compensation flow results in extreme loss of instream habitat. A discharge of 1.3 m3 s−1 ensures that water is maintained over most of the channel area at sites representative of upper, middle and lower sections of the North Tyne. This discharge lies above breaks in slope of respective site discharge versus wetted area curves; thus, disproportionate increases in discharge would be needed to increase wetted area. Simulations using the Physical Habitat Simulation System (PHABSIM) suggest that the compensation flow provides between 50% and 90% of the maximum possible weighted usable area (WUA) for juvenile (0+) salmonids. 3. During HEP releases, juvenile salmonid habitat (WUA) apparently falls to between 20 and 40% of site maxima. Newly emerged juvenile fish (March and April) are most affected by HEP releases because they are relatively small (25 mm in length) and water temperatures are relatively low at this time of year. During March and April, critical near-bed displacement velocities for newly emerged fish may be exceeded across large parts (80%) of sites up to 8 km downstream from Kielder Reservoir; fish would either be displaced downstream or forced to relocate to flow refuge areas. 4. The availability of Atlantic salmon spawning habitat (WUA) at a key site is limited by the compensation flow; 1.3 m3 s−1 provides approximately one third of the habitat available at the optimum discharge (4 m3 s−1). At this site, a discharge of approximately 2 m3 s−1 is needed to ensure most of the bed is inundated by water. Regulation has reduced the duration of flows exceeding 2 m3 s−1 from 90 to 60% of the spawning season. 5. Simulations suggest that when discharge drops from 30 m3 s−1 to the compensation flow, up to 60% of the optimum spawning habitat available at the former discharge may be left stranded (dry). This could potentially lead to egg or alevin mortality. 6. PHABSIM simulations suggest that increasing the compensation flow to 4 m3 s−1 during the spawning period (November and December) is likely to increase the availability of suitable spawning habitat. Also, increasing the compensation flow to 2 m3 s−1 during the incubation period (January through March) would minimize redd stranding. Reductions in the number of HEP releases in March and April would limit the extent to which newly emerged fish are exposed to velocities that potentially displace them. Such changes to the Kielder release regime may have implications for water resource management. While it is important that the biological instream flow requirements of the North Tyne are incorporated into the Kielder operating policy, these should be integrated along with the need for channel maintenance flows, downstream water supply abstractions and HEP generation, as well as for transfers of water to other catchments. Copyright

The validity of the Gammarus:Asellus ratio as an index of organic pollution: abiotic and biotic influences.

In freshwaters. Gammarus spp. are more sensitive to organic pollution than Asellus spp. and the relative abundance of the two taxa has been proposed as a pollution index. We tested the validity of this by examining the relationship between the Gammarus: Asellus (G : A) ratio and (1) a suite of physico-chemical variables. (2) established biotic (average score per taxon, ASPT) and richness (species richness (S) and Ephemeroptera, Plecoptera and Trichoptera families richness (EPT family richness)) indices generated from the macroinvertebrate community. In addition, we investigated a suspected biotic interaction, predation, between Gammarus and Asellus. Both univariate and multivariate analyses showed that the G: A ratio was sometimes responsive to changes in parameters linked to organic pollution, such as BOD5 and nitrate levels. However, the G : A ratio also appeared responsive to variables not directly linked to organic pollution, such as conductivity and distance from source. There were significant positive correlations among the G : A ratio and the ASPT, S and EPT, indicating that changes in the relative abundances of Gammarus and Asellus were reflected in changes in the pollution sensitivity and richness of the wider macroinvertebrate community. A laboratory experiment revealed significant predation of Asellus aquaticus juveniles by Gammarus duebeni celticus adults, but no reciprocal predation. We propose that the G: A ratio may be useful as a crude measure of organic pollution that could supplement more complex indices in a multimetric approach to pollution monitoring or be used for monitoring individual sites, where a simple technique is required for monitoring purposes over a period of time. Also, we urge recognition of the possible role of biotic interactions among taxa used in the generation of pollution indices.

Sediment entrainment and depletion from patches of fine material in a gravel-bed river

characteristics at shear stresses ranging from 5 to 60 N/m 2 . Experiments demonstrate that patches of fine sediment control both the intensity and duration of bed load under hydraulic conditions characteristic of the early stages of floods. The experiments allow quantifying bed load at the entrainment threshold, providing the first empirical evidence that marginal bed load transport can be attributed to the mobilization of sediments from patches. Bed load transport was recorded consistently once shear stress exceeded 5N /m 2 . The experiments produced low bed load rates (<6 g/sm). Depletion of material in the patches occurred rapidly, with bed load rates and particle sizes decreasing after only 5 minutes. Combining flume and Helley-Smith data for the study reach, a breakpoint in the relation between shear stress and bed load rate was calculated to be around 30 N/m 2 . This represents the transition between bed load transport phases: below the breakpoint, transport occurs at a low rate and is composed predominantly of fine sediment from patches, but above it, much higher rates occur from across the reach as a whole. Hydraulic conditions at the threshold are those which occur during small, frequent floods (25% bankfull, flow equaled or exceeded 15% of time). This indicates that sediment entrainment from patches of fine material is a frequent process and the threshold change between bed load phases occurs regularly.

Invertebrate communities and hydrological variation in Cairngorm mountain streams

Macroinvertebrates, discharge and 16 chemical variables were monitored over a 14-year period in four small streams (catchment area <15 km2) in the Cairngorm mountains, Scotland. Canonical Correspondence Analysis (CCA) was used to assess relationships between invertebrates and environmental conditions on the day of sampling, average conditions over the preceding 1, 2 and 3-month periods and indices of hydrological and hydrochemical variation over preceding monthly intervals.CCA detected subtle inter-catchment differences in invertebrate community structure, with catchments separated along axes representing streamwater calcium, alkalinity and total organic carbon concentrations. Invertebrate communities varied seasonally, with spring, summer and autumn samples separated along CCA axes representing temperature, orthosilicate and discharge. Hydrochemically, spring was the most variable season, characterised by increased frequency of both high and low flow events and acid, snowmelt episodes. In two of the streams, invertebrate community structure varied more in spring than in other seasons.CCA ordinations using indices of hydrological and hydrochemical variation over preceding time periods were more successful (increased eigenvalues) at explaining temporal variation in invertebrate community structure than those using conditions on the day of sampling or average conditions over preceding time periods. For one of the catchments, 40% of the seasonal and between-year variation over the 14-year period could be explained by the frequency of high and low flow events, maximum and minimum water temperatures and acid episodes in the two months prior to the invertebrate samples being collected. The single most important flow parameter (longest CCA arrow) was the frequency of high flow events greater than three times the median discharge.No significant trends in invertebrate community composition were found in any of the streams over the 14-year period so, despite the apparent importance of hydrological and hydrochemical variation, communities appeared stable over the long-term.

An approach to assessing hydrological influences on feeding opportunities of juvenile atlantic salmon (Salmo salar) : a case study of two contrasting years in a small, nursery stream

This case study sought to examine how temporal variability in hydrological and hydraulic conditions might affect the feeding opportunities of juvenile Atlantic salmon in two hydrologically contrasting years of 2002 and 2003, which were characterised by high and low flows respectively. Firstly, measures of hydraulic influence were calculated to define what might be ecologically meaningful disturbance periods during high flows. Secondly, for identifying such periods, the parameter Critical Displacement Velocity (CDV) was derived from a river discharge time series as a first approximation of the amount of time in two hydrologically extreme years when fish foraging strategies for specific age classes of juvenile Atlantic salmon (Salmo salar) might be disrupted by flows. The CDV estimates the threshold velocity above which juvenile salmon are unable to hold station and it is dependent upon fish size and stream temperature. In the wet year 2002, the CDV was exceeded on 18% and 21% of days for 0+ and 1+ fish respectively. In 2003 these respective numbers fell to 6% and 15%. The data suggest that hydrological conditions during certain times of the year have the potential to affect foraging behaviour. This in turn might have implications for recruitment and growth rates for juvenile salmon in upland streams.

The relationship between sediment mobilisation and the entry of Baetis mayflies into the water column in a laboratory flume

Mass bedload movement is thought to play a key role in initiating stream invertebrate drift during extreme flood events. However, little is known of the importance of the shear of invertebrates from stone surfaces relative to their entrainment along with bed material at different discharges. In particular, it is unclear whether so-called ‘catastrophic drift’ only occurs once mass bedload movement, and hence entrainment of invertebrates, occurs. We investigated the relationship between the mobilisation and transport of bed sediments and the entry of Baetis mayflies into the water column in a laboratory flume. Experiments quantified the percentage of Baetis drifting at a range of discharges that mobilised between 0 and 95% of the flume-bed sediments. Control experiments quantified drift losses from sediment fixed to the bed of the flume, such that sediments were immobile even at the highest discharges. Drift losses increased with increasing discharge and velocity in the flume. Sediment mobility contributed significantly to drift (ANCOVA, p < 0.001), with consistently greater drift losses in mobile sediment experiments than in those with fixed sediment. The discharge which resulted in a loss of 100% of Baetis from the mobile sediment bed (discharge 30 l s−1) resulted in a loss of approximately 50% of individuals from the fixed bed. Results indicate that once bed sediments are mobilised, entry of Baetis into the drift is greater than expected from the shear of animals from stone surfaces alone. Thus, entrainment of animals along with sediment contributes significantly to drift at high flows. This implies that differences in bed stability between sites or streams, or temporal changes in sediment characteristics within a site, could influence patterns of drift.

Does diurnal temperature variability affect growth in juvenile Atlantic salmon Salmo salar

This study investigated the effects of diurnal temperature variability (>7° C) on the growth of 1+ year Atlantic salmon Salmo salar. Experimental manipulation of water temperature was used to simulate: (1) constant and (2) naturally varying thermal regimes with similar daily mean values. Data from two replicates of four treatments (two thermal and two feeding regimes) were collected over 6 months corresponding to the main spring to summer growth period. Fish growth was assessed at fortnightly intervals. Small but significant differences in mean fork length (L(F) ) and mass were observed between temperature treatments, with smaller, lighter fish under the variable temperature regime. The effects of temperature regime on growth were independent of food ration. At termination of the experiment, the median L(F) and mass of fish exposed to the variable temperature regime were estimated, respectively, to be 2· 6 and 8· 0% less than those under the constant regime. Given the relatively small differences in growth attributable to variable temperature regime in these experiments, it is suggested that mean daily temperatures are adequate to inform juvenile growth models for field-based studies.

A new method to identify the fluvial regimes used by spawning salmonids

Basin physiography and fluvial processes structure the availability of salmonid spawning habitat in river networks. However, methods that allow us to explicitly link hydrologic and geomorphic processes to spatial patterns of spawning at scales relevant to management are limited. Here we present a method that can be used to link the abundance of spawning salmonids to fluvial processes at the mesoscale. We show that the frequency of spawning activity at individual morphological units (riffles, pools, runs) is quantitatively related to a number of fluvial parameters. Of these, bankfull excess shear stress (xs) was the best predictor of spawning frequency. Results suggest that xs can be used to represent the fluvial regimes that spawning salmon are responsive to as well as to assess the likely impacts of altered flow regimes.